The invention concerns a pressure-sensitive recording material with microcapsules, which contain a color former that is prepared on the basis of a triphenylmethane leuco-dye and that is dissolved in a solvent, characterized in that the color former is dissolved in a solvent composed at least 80% of plant, animal or paraffin oils, and the microcapsules are formed of a wall material which is water-soluble or water-dispersible before the microcapsule formation.
Color formers based upon triphenylmethane leuco-dyes are described in German Offenlegungsschrift No. DE-OS 2750283. These color formers effect an excellent formation of color upon contact with acid reactants. In order to be able to place this class of color formers into pressure-sensitive recording materials, they are dissolved in suitable solvents, of which DE-OS No. 2750283 names partially hydrated terphenyl, alkylated naphthaline and dibutylphthalate. Thereafter, the dissolved color formers are enclosed according to known techniques in microcapsules. The mentioned solvents are occasionally viewed with distrust, causing skin irritations upon use of the carbon paper manufactured therewith. On the other hand, there are also objections in particular upon repeat preparation of microcapsule-containing papers having breaks in the paper fabric.
Accordingly, there exists a need to be able to substitute these solvents by such solvents which are less hazardous in this connection. European Patent No. EP-A-167900 suggests, in particular, alkylated products of diphenylmethanes, e.g. ethyl diphenylmethane or alkylated products of diphenylethanes, e.g. ethyldiphenylethane. This reference discloses, as typical color formers, crystal violet lactone and N-leuco-auramine. The availability of these solvents is not, however, always simple. Moreover, it is not certain that the requisite expectations can be fulfilled by these solvents, to the broadest extent of unobjectionableness.
The older German Offenlegungschriften Nos. DE-OS 2242910, 2251350, 2306454 and 2726782 indeed extensively mention as being unobjectionable, oil-like substances such as e.g. paraffin oil, soy oil and fish oil. However, these oils were previously only considered as fillers, the last three of the above-mentioned Offenlegungschriften disclosing an amount of 0 to 3 parts by weight per weight part of an expressly well dissolving means for the color former. Whereas DE-OS No. 2242910 mentions only the possibility of encapsulation of the oil, without going into the particulars of the solvent characteristics for the known color formers, DE-OS No. 2306454 expressly states that these oils, designated as diluting agents, display only slight practical value as solvents, and inhibit the development of color. Their function is to reduce the cost and influence the physical characteristics, such as viscosity or vapor pressure. Actual tests of the applicants have proven that disadvantages with regard to the color reaction arise when the amount of the fillers is selected to be greater than 20 to 30%, relative to the total solvent mixture
European Patent No. EP-A-24898 suggests a solvent mixture of aromatic hydrocarbons and ester components in determined ratio whereby such solvent mixtures can, advantageously, be cut at leas to 50% by weight, and only in exceptional cases with greater amounts, of inert diluting agents, such as e.g. mineral or plant oils.
Also European Patent No. EP-A-86636, in which among a plurality of high-boiling solvents, the aromatic hydrocarbons are given preference over oils of animal or plant derivation, as well as mineral oils, expresses the generally prevailing opinion that the naturally occurring oils of mineral, animal or plant basis, are less well suitable, since they, despite their known physiological unobjectionableness, are also known as poor dissolvers for the customary color formers, and their use in significant amounts results in less of a color forming behavior, in particular a decreased color intensity.